The 28th International Cosmic Ray Conference----------1

The Telescope Array Experiment: The Search for The
Clusters in The Northern Hemisphere Sky with A Large
Scintillator Array
M.Chikawa,1 M.Fukushima,2 K.Hashimoto,5 Y.Hayashi,3 N.Hayashida,2K.Hibino,4 K.Honda, 5 H.Kawai,11 S.Kawakami,4 T.Matsuda,6 T.Nakamura,7M.Ohnishi,2 H.Ohoka,2 N.Sakurai,2 M.Sasano,8 K.Tanaka,9 M.Teshima,10S.Yoshida,11 T.Yoshikoshi3 and TA Collaboration.
(1) Kinki University, Osaka 577-8502, Japan(2) Institute for Cosmic Research, University of Tokyo, Chiba 277-8582, Japan(3) Osaka City University, Osaka 558-8585,Japan(4) Kanagawa University, Yokohama, 221-8686, Japan(5) Yamanashi University, Kofu 400-8510, Japan(6) Institute of Particle and Nuclear Studies, KEK, Tsukuba 305-0801, Japan(7) Kochi University, Kochi 780-8520, Japan(8) Communication Research Laboratory, Tokyo 184-8795, Japan(9) Hiroshima City University, Hiroshima 731-3194, Japan(10) Max Plank Institute for Physics, 80805 Muenchen, Germany(11) Chiba University, Chiba 263-8522, Japan

Abstract
In addition to the existence of extremely high energy cosmic rays (EHECRs)
above GZK cutoff, the results of AGASA shows the clustering of EHECRs above
1019.6 eV. Confirming such clusters and identifying the corresponding astronomical object is crucial to understand the origin of EHECRs. The northern hemispheresky is ideal for this purpose since the deflection by the galactic magnetic field is
expected to be small and uniform compared to the southern hemisphere. In thispaper, we report a design of the large ground array placed at the center of the hybrid-TA experiment. The array has an acceptance 9 times larger than AGASA.
Using this detector, the cluster in the northern hemisphere will be searched withan angular resolution better than 1 degree.
1. Introduction
So far AGASA has obtained nice results about the existence of super-GZKparticles. The results suggest a uniform distribution in the arrival direction ofEHECRs and at the same time the existence of clustering in some directions (see2 -----------The 28th International Cosmic Ray Conference

Fig.1). The candidate astronomical objects should have a large enough volumeto accelerate the nucleons up to 1020eV orhigher, but we so far have not been able
to locate any such object within 50 Mpc
of the Earth as sources of those EHECRs
and clusters. This introduces serious
problems to understand the origin andacceleration mechanism of EHECRs.
Several models have been proposedto try to give answer for these issues,
such as a decay of super massive relicparticles or cosmic strings1, a breakdown
Fig.1 Distribution of spatial angle between twoof special relativity at the energy above
cosmic rays measured by AGASA. All possible
1020eV2 and so on. But none of themcombinations are included.
seems to be able to convince people.
Considering these problems we have keen interest to confirm the existence of super-GZK cosmic rays and especially their clustering.
2. Performance of Hybrid-TA Ground Array
The overall plan of hybrid-TA is presented elsewhere in this conference. Oneof the major aims of the hybrid-TA is to quickly settle the issues around the clustering of EHECRs with higher statistics. To achieve this target, we have decided toadd conventional scintillation detectors as a ground array in addition to the fluorescent detectors originally planned in the TA project. Here we would like to concentrate on presenting the design and the performance of the ground detector array.
Total 576 detectors are to be distributed in 24x24 matrix covering the area of 760
km2 in 1.2 km mesh. Its acceptance is 9 times larger than that of present AGASA.
Each detector is a plastic scintillator of 3 m2 area and 2 cm thickness. The scintillation light is collected to a photomultiplier using wave length shifter bars installed in a groove cut on the surface of the scintillator. We calculated the detection efficiency for the present arrangement; it is around 50% at 1018.5 eV and becomes full above 1019 eV.
The plastic scintillator mainly measures the electrons and positrons in the airshower. It is easier than measuring other components such as muons and hadrons,since the total number of electrons and positrons is roughly 10 times larger and nospecial preparation is required. Better statistics in terms of the number of charged
particles in the detector can be achieved with smaller detector size compared tothe water Cherenkov detector. In addition, the measurement of electrons and posi-The 28th International Cosmic Ray Conference----------3

trons are less affected by the primary composition and the details of the hadronicinteraction at UHE because nearly 90% of the total energy is converted to theelectromagnetic component near the shower maximum where the ground detector
samples the air shower. This means the plastic scintillator has inherently better
energy resolution and less systematic uncertainty compared to other types of particle detectors. It should also be noted that, starting as low as 1014 eV, a series ofenergy spectrum measurements has been made by the plastic scintillator array, including AGASA, and valuable experiences have been accumulated. The continuation and consistency is a good asset for establishing a reliable energy spectrum atextremely high energy.
We adopt two approaches to achieve a good angular resolution with the present ground detector system; One is to obtain a good timing standard for all the576 counters and another is to obtain a good timing resolution for the hitting timeof each counter, which is achieved by recording the time profile of all the charged
particles hitting the counter by the flash ADC. As the time standard, we found anaccuracy of 20ns can be rather easily achieved with ordinary GPS, and a better
time resolution can be expected with the differential GPS system or by some othermeans such as selecting the same set of satellites for all the counters. These techniques are now being tested. We eventually aim at reaching 10ns accuracy byadopting these methods.
The Flash ADC board developed for the ground array isshown in Fig.2. It has a 50 MHz
12-bit ADC, a comparator for
generating a local trigger, aCPU and a TCP/IP network interface. The connection to theGPS is made by a RS232 serial
port. With a conventional fast
timing system, one measures theFig.2 Prototype Flash ADC Board
timing of first arriving particle
at the front of the shower disk. The arrival direction of the air shower is determined by combining all the timing information and fitting it to the shape of theshower front. In such a case, there can be a large fluctuation in timing measurement, especially where the particle density is sparse, and achieving a good angularresolution is limited. Using the flash ADC system, a complete history of ~10µs
after the arrival of the shower front will be recorded with 20ns time resolution. We
then can observe the distribution of arriving particles with time and determine the4 -----------The 28th International Cosmic Ray Conference

structure of the shower disk. It will give us a great advantage to have better resolution in the angle measurement. We are expecting the accuracy of 1 degree can beachieved with the present FADC system.
In terms of the energy
resolution of present design,major contribution comes fromthe fluctuation in the samplingof shower particles. We presently estimate that the error forthe determination of primaryenergy is around 30% atFig.3 Time profile of 30 m2 scintillation counter signal ob1019.5eV and 25% at 1020eV. On
served 2 km away from the shower core of a large air shower
event (E=2x1020 eV) observed by AGASA.
the other hand, we expect thatthe systematic uncertainty of theenergy determination will improve because the time structure of the shower disk
will be better understood for the new array. An example is shown in Fig.3. Thistime profile is obtained by a large (30m2) scintillator near the core of AGASAEHECR event. The signals 4,6 and 8 µs after the shower front is considered to bedelayed neutrons associated with the shower. In the new flash ADC system, wecan explicitly exclude these signals from the energy calculation and improve thesystematics, though the energy shift of AGASA arising from these effects is estimated to be only 5%3.
The whole system will be operated stand alone using a solar panel power
generation. Since a large amount of detectors are required, stable and reliable detectors are essential. We believe the present system is robust and easy to deploy
and maintain in the desert.
3. References
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K.Hamaguchi, I.Izawa, Y.Nomura and T.Yanagida, Phys. Rev. D69(1999) 125009
V.Berezinsky, Nucl. Phys. Proc. Suppl. 81(2000) 311

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H.Sato and T.Tati, Prog. Theo. Phys. 47(1972) 1788
S.Coleman and S.L.Glashow, Phys. Rev. D59(1999) 116008

3.

M.Takeda et al., Astroparticle Phys. 19(2003) 447